The present invention relates generally to eddy current inspection and, more specifically, to eddy current probes for non-destructive testing of conductive materials.
Eddy current inspection is a commonly used technique for non-destructive testing of conductive materials for surface flaws. Eddy current inspection is based on the principle of electromagnetic induction, wherein a drive coil carrying currents induces eddy currents within a test specimen, by virtue of generating a primary magnetic field. The eddy currents so induced in turn generate a secondary magnetic field, which induces a potential difference in the sense coils, thereby generating signals, which may be further analyzed for flaw detection. In the case of a flaw in the test specimen, as for example, a crack or a discontinuity, the eddy current flow within the test specimen alters, thereby altering the signals induced in the sense coils. This deviation in the signals is used to indicate the flaw.
Generally, coils that are relatively small in size (for example on the order of about 0.1 to 1.0 mm in length) are used to achieve high resolution. For example, U.S. Pat. No. 5,315,234, Sutton, Jr. et al., entitled “Eddy current device for inspecting a component having a flexible support with a plural sensor array,” uses several sensing coils connected in series for sensing small flaws. The voltage output of this configuration relies on a signal difference from two differential sensing coils. Since the coils are made as identical as possible, external electromagnetic noise is eliminated. Another advantage of the differential sensor is its attenuation of the noise associated with small lift-off variations.
However, the eddy current probes described above are limited in their utility by the fact that a prior knowledge of crack orientation is required. Due to this directionality of differential eddy current probes, if more than one flaw orientation is anticipated, the test specimen must be repeatedly scanned in different orientations to detect the flaws. The repeated scanning makes this process laborious and time consuming.
Accordingly, it would be desirable to have an improved eddy current probe and inspection system for detecting cracks and other linear flaws with random orientations. In addition, it would be desirable for the improved eddy current probe to have a large coverage area and be sensitive to both small and long defects in the test specimen.